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Subordinated Debentures Subordinated Debentures, Additional Information (Details) $ in Thousands | Sep. 30, 2020 USD ($)
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First Commonwealth Capital Trust II [Member] |
Debt Instrument [Line Items] |
Debt Issuance Costs, Gross | $ 600
First Commonwealth Bank 2028 [Member] |
Debt Instrument [Line Items] |
Subordinated Debt Par Amount | 50,000
Debt Issuance Costs, Gross | 900
First Commonwealth Bank 2033 [Member] |
Debt Instrument [Line Items] |
Subordinated Debt Par Amount | 50,000
Debt Issuance Costs, Gross | 1,100
First Commonwealth Capital Trust III [Member] |
Debt Instrument [Line Items] |
Debt Issuance Costs, Gross | $ 500

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- Definition Subordinated Debt Par Amount + References No definition available. + Details
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Namespace Prefix: | fcf_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | us-gaap_DebtInstrumentLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Amount, before accumulated amortization, of debt issuance costs. Includes, but is not limited to, legal, accounting, underwriting, printing, and registration costs. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 30 -Section 45 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=119993939&loc=d3e28555-108399 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Details
Name: | us-gaap_DebtInstrumentAxis=fcf_FirstCommonwealthCapitalTrustIIMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_DebtInstrumentAxis=fcf_FirstCommonwealthBank2028Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_DebtInstrumentAxis=fcf_FirstCommonwealthBank2033Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |